Handrail/Guardrail Embedment 2

[adamt83]

I've searched the several threads on this topic but haven't came to a clear understanding of the design of the embedment of handrail/guardrail in concrete. I realize that there is some debate as to whether core drilling posts in concrete slabs or retaining walls is the ideal installation over other alternatives (base plates, cast-in-place sleeves such as EZ Sleeve, etc.), but in my experience embedment of steel, stainless, or aluminum (with coatings) posts in concrete with non-shrink grout is commonplace. Can someone clarify the procedure to calculate the required embedment depth? Or even point me to some resources? Thanks!

 

[DaveAtkins]

I have never seen a resource on this, but I just make sure the forces are in equilibrium. At the base of a guardrail post, there will be a lateral force and a moment. These must be resisted by bearing pressures within the core.

DaveAtkins

 

[Ron]

Resolve the moment at the base and apply the force couple in a triangular distribution, then iterate the required depth. It should work out to be about 2-1/2 to 4 inches. Don't forget that you have to consider that you have to apply an upward load to the post as well as the lateral load (though not concurrently).....most codes state that you have to apply the loads (200 lb concentrated load or 50 plf) in any direction.

 

[cap4000]

Check out page 3 of this link.

http://www.ncree.org.tw/iwsccc/PDF/08 - Shahrooz.pdf

 

[a2mfk]

Thanks Cap and Ron, glad to know we have been doing it right but will recheck with the formal method... That is the first published resource I have seen on the subject.

Anecdotal - I have noticed several failures when the handrails are placed in concrete sidewalks or slabs very close to the edge, failed via a shear crack where the edge of the concrete spalled completely off. If its a structural slab and you can call out for an "insurance" rebar at that edge, its always nice

 

[adamt83]

Thanks for the replies guys. However, I'm still kinda confused. I can resolve the moments for the various load conditions no problem (assuming the rail post is a cantilever beam fixed at the base in the concrete), but I'm not clear on exactly how to apply the triangular distribution and iterate the req'd depth as Ron mentioned. I appreciate Cap's resource, but the equations presented on page 3 of the pdf for embedment seem to discuss beams w/ flanges; not sure how to modify for a round or square handrail post. Thanks again for the help.

 

[DaveAtkins]

The moment must be resolved into a couple, with the two forces being located at the centroids of the equal and opposite triangular bearing pressures. The horizontal force causes a uniform bearing pressure. Superimpose the uniform bearing pressure on the triangular bearing pressures.

DaveAtkins

 

[paddingtongreen]

I don't like to do this, I've had to fix too many where the rain has slipped into the joint, either rusting the pipe or sleeve if there is one, or freezing and breaking the concrete.

If I'm stuck with using a core, I use two opposing, rectangular bearing stress blocks, one from the top to the halfway point, the other from the halfway point to the bottom. This makes my moment arm half the depth of the core. It is tempting to use triangular blocks and make the moment arm two thirds of the depth, but that assumes unyielding pipe and is unconsevative.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[adamt83]

I'm having problems visualizing the stress blocks or triangular distributions that you guys mentioned. If someone has time and the means, a brief sketch might clear this up for me. Thanks again.

 

[slickdeals]

See attached. Hope this makes sense.

 

[slickdeals]

And yes, I got lazy. This is not how I do "real" sketches. It's Friday evening.

 

[DaveAtkins]

slickdeals,

The sketch shows resistance to the moment, but not the lateral load.

paddingtongreen,

I think a triangular bearing assumption is more realistic, and conservative.

DaveAtkins

 

[adamt83]

Thanks for the sketch. Assuming that the sketch is correct (rect. dist. instead of triangular) and calling the resultant of each stress block R, then M = R*D/2, correct? M is a known, but is R a function of the compressive strength of the grout? Thanks again; maybe its just too late on a Friday and I can't think anymore...

 

[Ron]

Try this....

 

[Lion06]

Guys, don't forget that there is a net lateral force, too, not just a moment. I do something similar to Ron, just a little different. I'll post a sketch when I can.

 

[adamt83]

Thanks Ron! That clears things up alot. For the lateral force, you would just do the same thing, but with a uniform loading, correct? Also, how do you know that for in this case the stress block area is E/3?

 

[Ron]

adamt83...the E/3 is a depth assumption simply because that is the most heavily stressed part of the triangle. Depending on contact area, there might be less stress but this is relatively conservative.

Same for lateral force, though it can be argued that a smaller stress block applies for the shear area. In actuality, the conservative approach would be to consider single shear through the concrete at the centerline of the post, extending E/3 downward, and to the outside edge of the concrete. Interior doesn't matter since the area of concrete in shear is way too large to be of consequence.

 

[racookpe1978]

OK, thanks to all for the data.

But when I build handrails into a slab or existing concrete sidewalk, I'm limited since the "base" is already solid.

What " works" - but I emphasize! not analyzed - is to core drill the sidewalk all the way through the concrete to dirt. If a slab, go down no more than 6 inches - since the core drill jams out any deeper. I've had no problems with holes only 4 inches deep, but prefer to go further if the sidewalk allows it.

On porches or wlakways in the open air, sometimes I can only go 3 inches deep before I bore through. If so, then I have to fill/plug/block off the bottom of the hole so the concrete doesn't just slide through the opening.

Plan on a 1-1/4 dia pipe (or equal) as the vertical, and get a 3.5 inch core drill. 4 dia if you can find one. The bigger the hole the easier it is to force the grout/concrete around the pipe without making a mess. Build and install the handrail pipe so it goes 5+ inches deeper than the surface, but doesn't hit any dirt.

Pour/insert/tamp the "expanding" grout/concrete around the post so you slightly over-fill the hole to force water runoff.

Keep the post supported overnight. Test the top of the handrail at 200 lbf the next day.

Note again: This is not a designed solution, but a construction "solution" report.

 

[Ron]

racookpe1978...I would not test the post to full load the next day. Most grouts are not that good!

 

[paddingtongreen]

Ron, your diagram presupposes that there is pressure on the pipe before the moment is applied, otherwise you could not have pressure on both sides simultaneously. I think your triangles should not overlap.

Michael.
Timing has a lot to do with the outcome of a rain dance.